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It's against Adsense ToS so I wouldn't risk it, the one you saw is taking a big risk with their account. From Adsense Program Policies:

Ad Placement

Publishers are encouraged to experiment with a variety of placements and ad formats. However, AdSense code may not be placed in inappropriate places such as pop-ups, emails or software. Publishers must also adhere to the policies for each product used.

Google ads, search boxes or search results may not be:

Integrated into a software application of any kind, including toolbars.

Displayed in pop-ups or pop-unders.

Placed in emails, email programs, or chat programs.

Obscured by elements on a page.

Placed on any non-content-based page. (Does not apply to AdSense for search or mobile AdSense for search.)

Placed on pages published specifically for the purpose of showing ads.

Placed on pages whose content or URL could confuse users into thinking it is associated with Google due to the misuse of logos, trademarks or other brand features.

Placed on, within or alongside other Google products or services in a manner that violates the policies of that product or service

If I can't be a good example, I'll just have to be a terrible warning...